The objective of this study is to isolate and characterize the glucocorticoid receptor from a hormonally sensitive target organ, the lactating mammary gland. Our preliminary studies have shown lactating goat mammary tissue to be a good source of this protein. We have developed purification procedures that enable us to prepare a highly purified glucocorticoid receptor. The purified glucocorticoid receptor is being characterized physiochemically and the topography of the steroid binding site is being studied by the use of radioactive affinity-labeling steroid derivatives and site-specific protein modifying agents. Binding of the purified "activated" steroid-receptor complex to DNA-cellulose or nuclei isolated from target organs will be utilized to identify physiologically important cytoplasmic steroid binding macromolecules. The effect on nuclear binding of modification of specific amino acid residues in the receptor will be studied in order to identify nuclear attachment sites of the receptor protein. Additionally, the purified receptor will be used for the production of an antiserum that may be helpful in the development of sensitive immunoassays to detect steroid hormone receptors in biopsy specimens from women with breast cancer.